Infinitely-variable transmission for a vehicle

ABSTRACT

An infinitely-variable transmission system is disclosed. The system comprises an input shaft ( 10 ) and an output shaft ( 14 ). The transmission is operative to transmit rotational drive between the input shaft ( 10 ) and the output shaft ( 14 ). The transmission includes a variator ( 20 ) that can transmit drive at a continuously variable ratio between a minimum variator ratio and a maximum variator ratio. The transmission can operate a low-speed regime and in a high-speed regime. The transmission is operative in the low-speed regime, at a neutral low regime variator ratio, the transmission is in geared neutral, in which, the output of the transmission is stationary irrespective of the speed of the input of the transmission and at a maximum low regime variator ratio, the output of the transmission is driven from the input of the transmission at a maximum low-regime transmission ratio, In the high-speed regime, at a minimum high regime variator ratio, the output of the transmission is driven from the input of the transmission at a minimum high-regime ratio and at a maximum high regime variator ratio, the output of the transmission is driven from the input of the transmission at a maximum high-regime transmission ratio.

This invention relates to an infinitely-variable transmission (IVT) fora vehicle. It has particular, but not exclusive, application to vehiclesthat must frequently reverse their direction of drive and have anefficient launch characteristic in both forward and reverse directions,a characteristic of loading vehicles, such as a back-hoe loader, wheelloader or a fork-lift.

A back-hoe loader requires a transmission that has drive characteristicsthat are quite unusual. During loading operations, the direction ofdrive is changed frequently, and high torque must be delivered to thedrive wheels in either direction while the vehicle is stationary ornear-stationary. Loading operations typically take place at low speed.However, the transmission must also provide a high-speed drive (whichneed be in one direction only) to allow the vehicle to be moved fromplace to place, for example on a public road.

A transmission for such a vehicle can be successfully implemented usinga transmission that drives through a torque converter and a multi-speedgearbox. Typically, the multi-speed gearbox has a plurality of forwardand reverse speeds. However, at low speeds (where the transmission willoperate for much of the time) this type of transmission suffers from thesignificant disadvantage that the torque converter cannot be locked up,and so will cause significant power losses, with the result that thefuel consumption of a vehicle equipped with such a transmission is high.

An aim of this invention is to provide a transmission system thatprovides drive characteristics that provide a low-speed, high torquedrive in forward and reverse directions without the with power lossesassociated with existing drives that incorporate a torque converter.

To this end, from a first aspect, this invention provides a transmissionsystem comprising an input shaft and an output shaft, the transmissionbeing operative to transmit rotational drive between the input and theoutput shaft, the transmission including a variator that can transmitdrive at a continuously variable ratio between a minimum variator ratioand a maximum variator ratio, which transmission can operate a low-speedregime and in a high-speed forward regime, the transmission:

operative in the low-speed regime, at a neutral low regime variatorratio, the transmission is in geared neutral, in which, the output ofthe transmission is stationary irrespective of the speed of the input ofthe transmission and at a maximum low regime variator ratio, beinggreater than the neutral low regime variator ratio, the output of thetransmission is driven from the input of the transmission at a maximumlow-regime transmission ratio; and

operative in the high-speed regime, at a minimum high regime variatorratio, the output of the transmission is driven from the input of thetransmission at a minimum high-regime ratio and at a maximum high regimevariator ratio, being greater than the minimum high-regime ratio, theoutput of the transmission is driven from the input of the transmissionat a maximum high-regime transmission ratio, being greater than theminimum high regime transmission ratio.

Optionally, it is operable at a transmission ratio higher than themaximum low-regime transmission ratio only in the high-speed regime.

In the above statement, and throughout this specification, “gearedneutral” is a condition in which the output of the transmission isstationary irrespective of the speed of the input of the transmission,this being achieved by suitable gearing, that causes two rotationalspeeds to be combined in a (typically epicyclic) gearset to create astationary output. The “transmission ratio” is the ratio of the speed ofthe output of the transmission system to the speed of the input of thetransmission system (the input being connected to an engine). The“variator ratio” is the ratio of the speed of the output to the speed ofthe input of the variator, where the variator is connected to theengine. When a ratio is described using words such as “high”, “maximum”,“low”, “minimum” or others that describe the value of the ratio, thisshould be taken to mean the absolute value of the ratio, ignoring thesign of its value, unless otherwise stated.

It will be clear that the variator must be swept from the maximum lowregime variator ratio to the minimum high regime variator ratio duringthe transition from the low-speed to the high-speed regime, and duringthe time that the transition is taking place, drive cannot be passedfrom the input to the output of the transmission without clutch slip andassociated power loss and wear. This is normally considered to be adisadvantage that would negate some of the benefits of acontinuously-variable transmission. However, in arriving at the presentinvention, it was realised that for some applications, a regime changeneed not be synchronous, provided that it does not take place whencontinuous torque delivery is required, as during loading operation, andthat the removal of the requirement for a synchronous regime changecould be advantageous in its effect on various aspects of the design ofthe transmission.

This is in contrast to a conventional CVT with a synchronous regimeshift, in which the variator ratio sweeps in opposite directions in thetwo regimes to achieve the same sense of change in the transmissionratio. For example, in a low regime, an increase in the variator ratiowill cause a decrease in the transmission ratio, while in a high regime,an increase in the variator ratio will cause an increase in thetransmission ratio. Such a transmission can be configured such thatthere is no change in the variator ratio or the transmission ratio as aresult of the regime change, which allows drive through the transmissionto be maintained during the regime change.

A transmission embodying the invention does not suffer from the powerlosses associated with a torque convertor, so can give performance equalto that obtained from a conventional transmission, but with reduced fuelconsumption so reducing operating costs. A lower-powered engine mightalso be used without loss of performance, so saving on costs in themanufacture of a vehicle and contributing to further gains in fuelefficiency.

Typically, the maximum low-regime transmission ratio is substantiallyequal to or greater than the minimum high-regime transmission ratio. Inthis way, despite the regime change not being synchronous, there is nostep change in the transmission ratio when regime change occurs.

A transmission embodying the invention typically comprises an epicyclicgear set. In the low-speed regime (and typically, the low-speed regimeonly), the epicyclic gearset is operative to reduce the transmissionratio and thereby provide a stationary, geared neutral output when thevariator is at the neutral low regime variator ratio and thetransmission is in the low-speed regime. Typically, such a transmissionincludes a low-speed clutch that is engaged during operation in thelow-speed regime to operatively couple the epicyclic gearset to thevariator and that is disengaged in the high-speed regime to operativelydecouple the epicyclic gear set from the variator. Typically, such atransmission typically further includes a high-speed clutch that isengaged during operation in the high-speed regime to couple the outputof the variator to the output of the transmission, and that isdisengaged during operation in the low-speed regime to decouple theoutput of the variator from the output of the transmission.

As is known in transmissions of this type, reversal of the direction ofthe output can be achieved in the low-speed regime by sweeping thevariator through the neutral low regime variator ratio to a lowervariator ratio. This has the advantage of providing a seamless change indrive direction. However, in embodiments of this invention, it may bepreferable to provide reversal of the direction of operation byselectively connecting or disconnecting a gearset into the drive paththrough the transmission. This can reduce the required spread of thevariator ratio to achieve the required overall spread of thetransmission ratio, with consequential benefits of reduced power lossesand gains in durability.

In embodiments of the invention, in the high-speed regime, all power(subject to losses) passing through the transmission passes through thevariator. Alternatively, in the high-speed regime, part of the powerpassing through the transmission bypasses the variator.

The variator may be ratio-controlled (that is, the variator ratio iscontrolled directly by an external control system) but is preferablytorque-controlled (that is, the variator ratio changes in response tothe torque on the input and the output of the transmission).

A variator typically operates in a volume of traction fluid, which haswell-understood viscosity properties that make it suitable for thattransmitting drive within the variator. The variator other components ofthe transmission system may operate in a common volume of tractionfluid. Alternatively, other components of the transmission system mayoperate in a separate volume of gear oil. This may be advantageousbecause of the relatively lower cost of gear oil as compared withtraction fluid.

A variator for use in the present invention may be selected from knownvariators of substantially any configuration that are capable ofproviding the required variator ratio spread and torque capacity to meetthe requirements of any particular embodiment of the invention. However,advantage may be gained by selection of a particular type of variator.While embodiments are described with reference to toroidal variators,and in particular to full-toroidal variators, other arrangements, suchas semi-toroidal variators, friction cone variators, push beltvariators, to name but a few, might also be used. However, in somecircumstances, a specific choice of variator may be advantageous.

Most typically, the variator in an embodiment of the invention is afull-toroidal variator. The full-toroidal variator may, for example,comprise a two-cavity variator comprising a first driving surface and afirst driven surface defining a first toroidal cavity and beingcoaxially mounted for rotation about a variator axis, a first pluralityof rollers in driving engagement with the first driving and first drivensurfaces; a second driving surface and a second driven surface defininga second toroidal cavity and being coaxially mounted for rotation aboutthe variator axis and a second plurality of rollers in drivingengagement with the second driving and second driven surfaces; and acontrol assembly on which the rollers in the first cavity and therollers in the second cavity are rotatably mounted and which assembly isadapted to balance the reaction torque from the first cavity with thereaction torque from the second cavity. A full-toroidal variator inembodiments of the invention may comprise a driving surface mounted forrotation on an input shaft defining a variator axis and a driven surfacecoaxially mounted for rotation with the driving surface, the surfacesdefining a toroidal cavity and two rollers in the or each toroidalcavity in driving engagement with the driving and driven surfaces, atake-off drive operatively engaged with the driven surface and disposedradially of the variator axis whereby a radial contact forceperpendicular to and intersecting the variator axis is generated andwherein the rollers are located such that the points of contact of therollers with the driving and driven surfaces at one particular ratiowithin the operating range of the variator generally lie in a planewhich is substantially perpendicular to the direction of the contactforce.

A full-toroidal variator in embodiments of the invention may comprise adriving and a driven disc having a variator axis, a plurality of pairsof contacting rollers interposed between said discs and the discs beingurged into contact by an applied end-load force, each of the rollershaving a first rolling surface by which it contacts the other roller ofthe pair and a second rolling surface by which each roller contacts thetoroidal surface of the corresponding disc, each roller is mounted on asupporting axle about which it can rotate; the rotational axes of therollers in a pair are supported in a plane or planes that contain thetwo points where the rollers of the pair contact the discs; at least oneof the rollers in each pair is adapted to be moved to adopt a stableposition within said plane by the reactionary force exerted on it by theother roller of the pair.

From a second aspect, this invention provides a vehicle that includes atransmission system embodying the first aspect of the invention. Thisaspect of invention has particular, but not exclusive, advantages whenthe vehicle is a loading vehicle, such as a backhoe loader, a wheelloader or a fork lift.

An embodiment of the invention will now be described in detail, by wayof example, and with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic representation of a transmission system being afirst embodiment of the invention;

FIG. 2 is a diagrammatic representation of the scheme of thetransmission of FIG. 1;

FIG. 3 is a view of the layout of the components of the transmission ofFIG. 1;

FIG. 4 is a graph that shows the relationship between variator ratio andvehicle speed of a vehicle equipped with the transmission of FIG. 1;

FIG. 5 is a perspective view of a two-cavity variator suitable for usein the transmission system being part of the embodiment of FIGS. 1 to 3;

FIG. 6 is a side view of a single-cavity variator suitable for use inthe transmission system being part of the embodiment of FIGS. 1 to 3;

FIG. 7 is a diagrammatic sectional view of a two-cavity, twin rollervariator suitable for use in the transmission system being part of theembodiment of FIGS. 1 to 3;

FIG. 8 is a schematic elevation, partly in section, of the variator ofFIG. 7;

FIG. 9 is a view similar to that of FIG. 8, showing the rollers inanother position;

FIG. 10 is a graph that illustrates reduction in power losses that canbe achieved through use of an embodiment of the invention in a loadingvehicle; and

FIG. 11 shows schematically an alternative configuration of embodimentsof the invention that employ power splitting between paths through thetransmission that include and that bypass the variator.

With reference to FIGS. 1 to 3, a transmission system comprises an inputshaft 10 that is connected for use to an output shaft of an engine of avehicle. A front output shaft 12 is connected for use to a final driveunit at the front of a vehicle, and a rear output shaft 14 is connectedfor use to a final drive unit at the front of a vehicle. An auxiliaryoutput shaft 16 is driven directly from the input shaft 10. Theauxiliary output shaft 16 is used to drive ancillary components of thevehicle, such as hydraulic pump. A first spur gear 18 is carried on theinput shaft 10.

The transmission system includes a full-toroidal variator 20. This is ofknown construction, so only its main components will be describedbriefly here. The variator includes two input discs 22 and two outputdiscs 24, each having a part-toroidal recess, whereby a toroidal cavityis formed between each input disc 22 and each output disc 24. All discshave a common rotational axis, which, in this embodiment, is coaxialwith the input shaft 10, which is coupled for rotation with the inputdiscs 22. Each cavity contains a set of rollers 26 that can transmitdrive between the input discs 22 and the output discs 24 through amedium of traction fluid. The ratio R_(v) of the speeds of the inputdiscs 22 and the output discs 24 is dependent upon the angle of tilt ofthe rollers 26 with respect to the discs 22, 24. An output gear 28 isprovided between the output discs 24 and is fixed for rotation withthem.

The transmission system further includes an epicyclic gearset 40, which,as is conventional, includes a sun wheel 42, an annulus 44, and a planetcarrier 46 on which is carried a plurality of planet wheels 48. The sunwheel 42 is fixed for rotation with a sun wheel shaft 50. Also fixed onthe sun wheel shaft 50 is a second spur gear 52, the second spur gear 52being in mesh with the first spur gear 18, with a ratio R₁. A planetgear 54 is carried on, and fixed for rotation with, the planet carrier46. The ratio of the speed of the sun wheel 42 to the planet carrier 54will be denoted R_(ce).

Third and fourth spur gears 60, 62 are carried on and fixed for rotationwith a first intermediate shaft 64. The third spur gear 60 is in meshwith the output gear 28 of the variator 20 with a ratio R₂, and thefourth spur gear 62 is in mesh with the planet gear 54.

The planet gear 54 drives an input gear 66 of a high-regime clutch 68with a ratio R₃. The high-regime clutch 68 can selectively connect ordisconnect drive from its input gear 66 to the rear output shaft 14.

The annulus is connected to drive a second intermediate shaft 70. Thesecond intermediate shaft 70 drives an input to a low-regime clutch 72and a reverse clutch 74. The low-regime clutch 72 can selectivelyconnect or disconnect drive from the second intermediate shaft 70 to ahigh-regime output gear 78. The high-regime output gear 78 is in meshwith a fifth spur gear 80, with a ratio R₄, that is carried on and fixedfor rotation with the rear output shaft 14. The reverse clutch 74 canselectively connect or disconnect drive from the second intermediateshaft 70 to a reverse intermediate gear 82, which is connected throughan idler gear 84 to a sixth spur gear 88, with an overall ratio R₅ thatis carried on and fixed for rotation with the rear output shaft 14.

The sixth spur gear 88 drives an input gear 90 to a front-drive clutch92. The front-drive clutch 92 can selectively connect or disconnectdrive from its input gear 90 to the front output shaft 12.

The transmission can operate in three regimes: low-speed forward,reverse, and high-speed forward. Operation of the transmission in thesethree regimes will now be described.

At all times, the sun wheel 42 is driven by the input shaft 10 throughthe first and second spur gears 18, 52 in the opposite direction to theinput shaft 10. The first and second spur gears 18, 52 provide areduction ratio between the input shaft 10 and the sun wheel 42. At alltimes, the planet carrier 48 is driven from the output of the variatorthrough the third and fourth spur gears 60, 62, and the planet gear 54in the opposite direction to the input shaft 10. In this mode, theoutput speed na of the epicyclic gearset is given by:

$\begin{matrix}{{n_{a} = {n_{p}\lbrack {1 - {( {\frac{n_{s}}{n_{p}} - 1} )\frac{z_{s}}{z_{a}}}} \rbrack}},} & (1)\end{matrix}$

where np and ns are, respectively, the input speeds of the planetcarrier 48 and the sun wheel 42, and za and zs are, respectively, thenumber of teeth on the annulus 44 and the sun wheel 42.

In the low-speed forward regime, the low-regime clutch 72 is engaged,and the reverse clutch 74 and the high-regime clutch 68 are disengaged.The output shaft 14 is driven by the annulus 44 through the low-regimeclutch 72, the high-regime output gear 78 and the fifth spur gear.

In the low-speed forward regime, when the variator 20 is set to aneutral variator ratio (which may or may not be its lowest ratio), theoutput of the transmission as a whole is stationary—that is, it is ingeared neutral. It will be seen from equation (1) above that:

$\begin{matrix}{n_{a} = {{0 \leq \frac{n_{s}}{n_{p}}} = {\frac{z_{a}}{z_{s}} + 1}}} & (2)\end{matrix}$

Therefore, geared neutral is achieved by suitable selection of theratios of the first and second spur gears 18, 52; the ratios of thethird and fourth spur gears 60, 62 and the planet gear 54; and theminimum ratio of the variator 20 to satisfy equation 2.

Assume now that the variator ratio 20 is increased. This results in alinear increase in the value of np, with the transmission ratio and therate of increase being determined by the value of za/zs. In thelow-speed forward regime, the transmission ratio of the transmission canbe varied continuously as the variator 20 is moved from the ratio atwhich geared neutral is achieved (the neutral low regime variator ratio)through its entire ratio spread to the maximum low-regime variatorratio, at which the transmission ratio is r_(low,max).

In the reverse regime, the reverse clutch 74 is engaged, and thelow-regime clutch 72 and the high-regime clutch 68 are disengaged. Theoutput shaft 10 is driven by the annulus 44 through the reverse clutch74, the reverse intermediate gear 82, the idler gear 84 and the sixthspur gear 88. The idler gear 84 serves to reverse the direction of driveto the rear output shaft 14. In this example, the transmission ratio inreverse is higher than in the low-speed forward regime. This is achievedby selecting the intermediate gear 82 to be larger than the high-regimeoutput gear 78.

In the high-speed forward regime, the high-regime clutch 68 is engaged,and the reverse clutch 74 and the low-regime clutch 72 are disengaged.The output shaft 14 is driven from the output of the variator 20 drivingthrough the third and fourth spur gears 60, 62; the planet gear 54, theplanet gear 54 through the input gear 66 and the high-regime clutch 68.The annulus 44 is free to rotate, so the epicyclic gearset does notoperate as part of the drive train in this regime. Thus, a change in theratio of the variator 20 has a direct effect on the overall transmissionratio. The transmission ratio in the high-speed forward regime extendsbetween r_(high,min) and r_(high,mxz).

In order to change between the low-speed forward regime and thehigh-speed forward regime, drive must first be removed from the inputshaft. The low-regime clutch 72 is then disengaged, and the variator 20is swept back towards its low-ratio limit to a position in which thetransmission will have a transmission ratio r_(high,min)=r_(low,max).The high-regime clutch 68 is then engaged and power can once again beapplied to the input shaft 10. The relationship between engine speed androad speed in the three operating regimes is illustrated in FIG. 4.

The transmission can be operated in a two-wheel drive mode or afour-wheel drive mode by disengaging or engaging the front-drive clutch92.

Inherent in the nature of the invention is that the variator ratio hasto be ‘reset’ at the regime change point. Therefore, an option open tothe designer of an embodiment of the invention is to extend low regimeto and then not use the whole of the variator ratio spread in high-speedforward regime.

There will be a momentary break in the torque delivery to the wheels atthe low-to-high regime change point while the variator ratio is beingreset. For this reason it is considered desirable to ensure that theregime change will not occur during operating conditions in whichcontinuous torque delivery is required. For example, in a transmissionfor a loading vehicle such as a backhoe loader or a wheel loader, it isdesirable that the regime change take place at a speed greater than thatat which loading operations will take place. This might typically bearound 9 to 12 km/h, which corresponds to speeds achieved in 2^(nd) gearof a conventional multi-speed torque converter transmission.

Immediately following the change into the high-speed regime, the enddiscs 22 of the variator 20 are subject to their highest levels ofcontact stress as the variator ratio 20 is at a minimum and theprecession angle of the rollers 26 at its highest. Time spent at thiscondition could have a significant detrimental impact on durability ofthe variator 20. If the spread of the variator ratio (and hence thetransmission ratio spread) in the high-speed regime is reduced, whilestill having the maximum speed/transmission ratio at the maximumvariator ratio, then this area of peak stress can be avoided.

For example, taking the regime change to be at 9.0 km/h (from above) thevariator ratio/transmission ratio spread in high-speed regime would be40/9=4.44. If the variator ratio spread in the low-speed regime is 6.5and is symmetrical around the 1:1 position, then the variator ratiocapabilities are −0.39 to −2.55 (note that the negative sign simplyindicates that the variator input and output rotate in oppositedirections). Following the change into the high-speed regime, instead ofresetting the variator ratio to −0.39 it need only be reset to −0.57(i.e. −2.55/4.44). This is apparent from FIG. 4, in which the absolutevalue of the variator ratio at geared neutral (at A) is somewhat lessthan the transition to the high-speed forward regime (at B).

In order to operate, the full-toroidal variator of this embodiment mustbe provided with a volume of traction fluid. While this can also be usedto lubricate other components in the transmission, this isdisadvantageous since traction fluid may be more expensive thanconventional gear oil. Therefore, the present embodiment is constructedsuch that only the variator 20 operates in traction fluid, with theremaining components being separate and operating in conventional gearoil. This ensures that the amount of traction fluid required foroperation of the transmission is minimised, which is advantageous inthat it minimises cost. In addition, the efficiency with which gearsoperate in gear oil may be slightly better in gear oil than in tractionfluid.

FIG. 5 shows a two-cavity full-toroidal variator suitable for use inembodiments of the invention. The variator has a firsttoroidally-recessed driving disc 110, facing which is a first toroidallyrecessed (the recess not being shown) driven disc 112. A first toroidalcavity is defined between the opposing toroidally-recessed faces of thedriving and driven discs 110, 112. The driven disc 112 also has a secondtoroidal recess 111 on its opposite side providing a second toroidallyrecessed driven surface. A second toroidally-recessed driving disc 113is provided, defining a second toroidal cavity with the second drivensurface 111. Each of the discs 110, 112, 113 is located on a commonaxis, which will be referred to as the variator axis. The input shaft 10is coaxial with the variator axis. Each of the driving discs 110, 113 isfixed to the input shaft 10 for rotation with it. The driven disc 112 iscarried for rotation on the input shaft 10.

First and second rollers 114, 116 are mounted in the first toroidalcavity to transmit drive from the driving disc 110 to the driven disc112 with a variator ratio which varies as the first and second rollers114, 116 tilt. Third and fourth rollers 115, 117 are mounted in thesecond toroidal cavity to transmit drive from the driving disc 111 tothe driven disc 113 with a ratio which is variable by tilting the thirdand fourth rollers 115, 117.

The first and second rollers 114, 116 are rotatably mounted in a firstroller carrier 140. The first roller carrier 140 comprises a firstroller carriage 144, 146 for the first roller 114 and a second rollercarriage for the second roller 116. A second roller carrier 141comprises similar roller carriages for the third and fourth rollers 115,117. The rollers 114, 116 and 115, 117 are each mounted by means of arespective stub axle 142 that is rotatably mounted in a respectiveroller carriage, each being defined by opposed planar support plates144, 146. The mounting of the rollers is numbered only on one roller forillustrative purposes and in the interests of clarity. The rollers 114,116; 115, 117 are mounted on the carriers 140, 141 via sphericalbearings to provide the required degrees of freedom of movement. Thefirst roller carrier 140 carries the first and second rollers 114,116,and the second roller carrier 141 carries the third and fourth rollers115,117.

Each roller carrier 140, 141 comprises a respective cross-bar 148, 149which links the two roller carriages within the roller carrier 140, 141.Each roller carriage is mounted on the cross bar 148, 149 such that itcan pivot with respect to the cross bar about an axis that is normal toboth the variator axis and to the axis of rotation of the stub axle142—that is to say, tangential to the discs 110, 112, 113. Eachcross-bar is pivotally mounted, the pivot axis being parallel to thevariator axis and normal to the axis of rotation of the rollers 114,116;115, 117.

Each cross-bar 148, 149 is provided with an actuating arm 160, 161 whichprojects in a radial direction from the variator axis in a directiongenerally parallel to the axis of rotation on the associated rollercarriages on the cross-bars 148, 149. End portions of the arms 160, 161project out of a variator housing are have recess that is C-shaped, whenviewed along the variator axis, for direct mechanical engagement with acontrol linkage.

The control linkage 164 comprises a linking lever 166 mounted to pivoton a pivot bearing 168 about an axis that is normal to and intersectsthe variator axis. The pivot bearing 168 is carried on an output elementof an actuator 170. End portions of the linking lever 166, to oppositesides of its pivot axis are each received in the recess of a respectiveone of the arms 160, 161, whereby the linking lever 166 is operativelylinked to the carriers 140, 141. These end portions are part-sphericalin shape to enable them to move smoothly within the recesses. Thus, asthe linking lever 166 rotates about its pivot bearing 168, the arms 160,161 move the arms 160, 161 in opposite directions in respective planeperpendicular to the variator axis. Rotation of the linking lever 166will cease when the forces applied to it be the arms 160, 161 are equalin magnitude and opposite in direction.

The actuator 170 is a dual-acting hydraulic actuator in this embodiment.Its output element can be caused to move in a direction tangential tothe discs 110, 112, 113, carrying the linking lever 166 with it.

During operation of the variator, when the actuator 170 moves from acentral position (in which the discs are rotating about an axis that isnormal to the variator axis) its force is balanced by a reaction torquethat acts on the rollers 114, 116; 115, 117 about the variator axis. Thetorque on the first and the second rollers 114, 116 is in a directionopposite to that of the third and the fourth rollers 115, 117.Therefore, the forces on the opposite end portions of the linking lever166 act in opposite directions. As has been discussed, the linking lever166 will rotate until such forces are balanced, which implies that thereaction torque from one cavity will also be balanced by the reactiontorque from the other cavity.

The driven disc 112 is provided with teeth 130 on its circumference toform a spur gear, whereby drive may be taken through the third spur gear60. A gear force Fg is generated which may impart a bending force to thedriving shaft and cause the more distant parts of the driving surfaces110, 113 to bow or splay away from the driven surfaces 112, 111.

FIG. 6 shows a further variator suitable for use in embodiments of theinvention. In this embodiment, the variator has a single cavity formedbetween a toroidally-recessed driving disc 110 and a facing toroidallyrecessed driven disc 112. The discs are coaxial and rotate about avariator axis. Two rollers 114, 116 (116 not shown) are mounted in thetoroidal cavity defined between the opposing toroidally-recessed facesof the driving and driven discs 110, 112 to transmit drive from thedriving disc 110 to the driven disc 112 with a ratio which is variableby tilting the rollers 114, 116. The rollers rotate about a common axisthat will be referred to as the roller axis. An input shaft 10 iscoaxial with the variator axis. The driving disc 110 is fixed to theinput shaft 10 for rotation with it. The driven disc 112 is carried forrotation on the input shaft 10.

The driven disc 112 is provided with teeth 130 on its circumferencewhereby drive is taken to the third spur gear 60. A gear force Fg isgenerated which imparts a bending moment to the input shaft 10, whichcauses the more distant parts of the driving surface of the driving disc110 and the driven surface of the driven disc 112 to splay apart.

Each roller 114, 116 contacts the driving disc 110 and driven disc 112arranged in such a way that the force Fg is perpendicular to the twoplanes that pass through the roller-disc contact points when thevariator is at a −1.0 ratio (that is, when the roller axis isperpendicular to the variator axis). This is achieved by ensuring that aline joining the axis of rotation of the third spur gear 60 and thevariator axis is perpendicular to the roller axis.

With this orientation, for each disc 110, 112, the roller-disc contactpoints are located in a plane that passes through the neutral axis ofbending of the variator. This means that the distance between the twocontact points between each roller 114, 116 and the discs 110, 112 issubstantially invariant wen the shaft 10 bends, and any such variationthat does occur will affect each roller substantially equally. Thismeans that the normal contact forces between the discs 110, 112 and therollers 114, 116 are not substantially affected by the radial force Fgand each roller contact bears an equal proportion of an applied end-loadforce.

The rollers 114 and 116 may each be actuated by an actuator 170 or mayboth be actuated by a single actuation mechanism. Hydraulic actuatorsmay be employed to provide ratio control. The variator is preferablytorque-controlled.

FIG. 7 shows diagrammatically a further variation on a two-cavityvariator suitable for use in embodiments of the invention. As with theembodiment of FIG. 5, this variator has a driven disc 210, and first andsecond driving discs 201, 200. A first toroidal cavity is formed betweenthe first driving disc 201 and the driven disc 210, and a secondtoroidal cavity is formed between the second driving disc 200 and thedriven disc 210.

Instead of the disc-shaped rollers, that extend from the driving to thedriven discs, present in the previous embodiments, this variator hasseveral twin roller sets supported between driving and driven discs. InFIG. 7, one roller set includes first and second rollers 212 and 213,and three similar roller sets are also shown.

Each roller has an outer surface that has two distinct regions. Aconical region that presents an external, generally frusto-conicalrolling surface 214, and a spherical region that presents an external,generally part-spherical rolling surface 215. (Instead of being exactlyfrusto-conical, the conical region may be formed with a very large crownradius or with curved edges so as to reduce stress concentrations at theedges of the conical surface.)

Drive from the driving disc 201 is transmitted to the first roller 212of the roller set, thence to the second roller 213 of the roller set,which then drives the driven disc 210. The rollers 212, 213 in theroller set are arranged such that only their spherical regions makecontact with the discs and only their conical regions make contact witheach other.

Within each roller set, the rollers are carried on a roller carrier thatallows them to rotate about axes that have an angle fixed with respectto one another. The cone angle of the conical region, and the anglebetween the axis, is arranged such that, when clamped together along thevariator axis by the discs 201, 200, 210, the rotational axes of all ofthe rollers lie within a plane that passes through the variator axis butrun through the variator axis 216. The axes are displaced from eachother under the influence only of the conical surfaces and the clampingreactions. The degree of displacement is such that in at least oneposition, each roller (not necessarily simultaneously) experiences astate where the differential velocities across the contacting surfacesis less than 0.5% and where the tangent of the disc and roller surface210 at the centre of the point of contact, and the roller rotationalaxis 211, and the disc rotational axis 216 generally pass through thesame point 217.

The variator shown in FIGS. 9 and 10 has at least one pair of rollers.The rollers contact each other along a theoretical line the centre pointof which is indicated by m, which line remains always perpendicular tothe line AB passing through the centre-points of the contact lines ofthe two rollers and of each roller and its corresponding disc. The axesX_Y respectively of the two rollers of each pair always intersect or areconcurrent and are always situated in a plane containing the variatoraxis CD of the driving and the driven discs.

As has been stated, it is an important aim of this invention to providea transmission system for a vehicle that reduces power losses andthereby effects a saving in operating cost, as compared with aconventional transmission. FIG. 10 presents the predictions of a modelfor power losses of a conventional 5-forward/3-reverse step-changetransmission driving through a torque converter, and a transmissionembodying the present invention, as applied to a backhoe loader or awheel loader. Vehicle roadspeed is on the x-axis and power loss on they-axis in each gear. The traces labelled 1f to 5f are for the fiveforward speeds and the traces labelled 1r to 2r are for the threereverse speeds. The power loss for each individual speed in theconventional gearbox is plotted, as is the single-line plot thatrepresents power loss using an embodiment of the invention.

In FIG. 10, the hatched area represents the difference in power lossbetween the two transmission systems. This suggests that the embodimentof the invention is capable of achieving significant reduction in powerlosses as compared with a conventional transmission system.

In the embodiments described above, the regimes operate with thevariator in a direct mode. That is, all of the power flowing through thetransmission passes through the variator. As is well-known to thoseskilled in the technical field, at least two alternative arrangementsare possible within a transmission.

In a first alternative, which will be referred to as a “power split”,only a portion the total power passes through the variator, with theremaining following a direct mechanical path from the input to theoutput of the transmission, bypassing the variator. In this arrangement,the epicyclic gearset is disposed between the input to the transmissionand the variator.

In a second alternative, which will be referred to as “powerrecirculation”, a portion the total power passes through the variator ina direction from the output to the input, with power in excess of thetotal passing through the transmission following a direct mechanicalpath from the input to the output of the transmission, bypassing thevariator. In this arrangement, the epicyclic gearset is disposed betweenthe output of the transmission and the variator.

It is contemplated that embodiments of the invention might use at leastpowersplitting or power recirculation in the high-speed regime.Suitably, the direction of flow of power through the direct mechanicalpath and the variator is in the same direction.

One possible arrangement of a transmission that uses power splitting isshown in FIG. 11. This arrangement comprises a low-speed regimeepicyclic gearset 310 and a high-speed regime epicyclic gearset 312,each having an associated clutch 314, 316 that can connect the input tothe output of the associated epicyclic gearset. This arrangementprovides an epicyclic gearset on the input to the transmission when itis operating in the high-speed regime.

What is claimed is:
 1. A transmission system comprising: an input shaftand an output shaft, the transmission being operative to transmitrotational drive between the input and the output shaft, thetransmission including a variator that can transmit drive at acontinuously variable ratio between a minimum variator ratio and amaximum variator ratio, which transmission can operate a low-speedregime and in a high-speed forward regime, the transmission: operativein the low-speed regime, at a neutral low regime variator ratio, inwhich, the output of the transmission is stationary irrespective of thespeed of the input of the transmission and at a maximum low regimevariator ratio, being greater than the neutral low regime variatorratio, the output of the transmission is driven from the input of thetransmission at a maximum low-regime transmission ratio; and operativein the high-speed regime, at a minimum high regime variator ratio, theoutput of the transmission is driven from the input of the transmissionat a minimum high-regime ratio and at a maximum high regime variatorratio, being greater than the minimum high-regime variator ratio, theoutput of the transmission is driven from the input of the transmissionat a maximum high-regime transmission ratio, being greater than theminimum high regime transmission ratio; and wherein the change betweenthe low-speed regime and high-speed regime is not synchronous.
 2. Atransmission according to claim 1, in which the maximum low-regimetransmission ratio is substantially equal to or greater than the minimumhigh-regime transmission ratio.
 3. A transmission according to claim 1,further comprising an epicyclic gear set.
 4. A transmission according toclaim 3, that includes a low-speed clutch i) that is engaged duringoperation in the low-speed regime to operatively couple the epicyclicgearset to the variator and that is operatively disengaged in thehigh-speed regime to operatively decouple the epicyclic gearset from thevariator or ii) that selectively connects or disconnects the epicyclicto the transmission output.
 5. A transmission according to claim 1, thatincludes a high-speed clutch that is engaged during operation in thehigh-speed regime to couple the output of the variator to the output ofthe transmission, and that is disengaged during operation in thelow-speed regime to decouple the output of the variator from the outputof the transmission.
 6. (canceled)
 7. A transmission according to claim1, further comprising a gearset that can be selectively connected intoor disconnected from the drive path at least in the low-speed regime, tocause the output of the transmission to be reversed.
 8. A transmissionaccording to claim 1, in which, in the high-speed regime, all powerpassing through the transmission passes through the variator.
 9. Atransmission according to claim 1, in which, in the high-speed regime,part of the power passing through the transmission bypasses thevariator.
 10. A transmission according to claim 1, in which the variatoris torque-controlled.
 11. (canceled)
 12. A transmission according toclaim 1, in which the variator operates in a volume of traction fluid,and other components of the transmission system operate in a separatevolume of gear oil.
 13. A transmission according to claim 1, in whichthe variator and other components of the transmission system operate ina common volume of traction fluid.
 14. A transmission according to claim1, in which the variator is a full toroidal variator.
 15. (canceled) 16.(canceled)
 17. (canceled)
 18. A vehicle that includes a transmissionsystem according to claim
 1. 19. A vehicle according to claim 23, thatis operable to perform loading operations.
 20. A vehicle according toclaim 23, that is one of a backhoe loader, a wheel loader, or a forklift.
 21. A transmission according to claim 1, wherein the low speedregime only, is operative to allow the neutral transmission ration to beachieved within the ratio speed of the variator in low-speed regime. 22.A transmission according to claim 1, in which the variator has twocavities with two rollers disposed in each cavity.
 23. A transmissionaccording to claim 1, in which the transmission is operable at atransmission ratio higher than the maximum low-regime transmissionration only in the high-seed regime.
 24. A low-speed, loading vehiclethat includes: a transmission system comprising: an input shaft and anoutput shaft, the transmission being operative to transmit rotationaldrive between the input and the output shaft, the transmission includinga variator that can transmit drive at a continuously variable ratiobetween a minimum variator ratio and a maximum variator ratio, whichtransmission can operate in a low-speed regime the transmission beingoperative in the low-speed regime, at a neutral low regime variatorratio, in which, the output of the transmission is stationaryirrespective of the speed of the input of the transmission and at amaximum low regime variator ratio, being greater than the neutral lowregime variator ratio, the output of the transmission is driven from theinput of the transmission at a maximum low-regime transmission ratio andin which the transmission ratio in low regime sweeps up as the variatorratio sweeps up to provide a low-speed, high torque drive in forward andreverse directions.
 25. A low-speed loading vehicle according to claim24, in which the transmission, in the low-speed regime only, isoperative to allow a geared neutral transmission ratio to be achievedwithin the ratio spread of the variator in low-speed regime.
 26. Alow-speed loading vehicle according to claim 24, which is a fork lift.27. A vehicle according to claim 24, further comprising a gearset thatcan be selectively connected into or disconnected from a drive path ofthe transmission in its the low-speed regime, to cause the output of thetransmission to be reversed.